Operation of jet engines to decrease boric oxide deposits



E. BECKER Oct. 31, 1961 OPERATION OF JET ENGINES TO DECREASE BORIC OXIDE DEPOSITS Filed June 18, 1959 I INVENTOR. [ugene fiecker BY United States ate r lgg 3,006,141

3,006,141 OPERATION OF JET ENGINES T DECREASE BORIC OXIDE DEPOSITS Eugene Becker, Grand Island, N.Y., assignor to 01m Mathieson Chemical Corporation, a corporation of Filed June 18, 1959, Ser. No. 821,321 2 Claims. (Cl. 6035.4)

This invention relates to a method of minimizing boron oxide deposits on surfaces within jet-type aircraft engines operating on a boron containing fuel. By boron containing fuel is meant a high-energy fuel such as a boron hydride, including diborane, the pentaboranes, and decaborane, organoboranes, including the lower alkyl penta boranes and lower alkyl decaboranes, and conventional hydrocarbon jet fuels containing the same. Lower alkyl pentaboranes can be prepared, for example, according to the method described in application Serial No. 546,803, filed November 14, 1955, now abandoned, of Jack R. Gould and John E. Paustian. Lower alkyl decaboranes can be prepared, for example, according to the method described in application Serial No. 557,634, filed January 6, 1956, to Joseph A. Neff and Edward J. Wandel.

The present invention can be applied to any of the three basic types of jet engines, i.e. the ram-jet, the turbo-jet, and the turbo-prop, although it has particular application to the turbo-jet and turbo-prop type engines, and will be further described as applied to a turbo-jet engine.

In the operation of a turbo-jet aircraft engine, air flows into the engine through the air entrance section and then into the compressor section, where it is usually compressed to a pressure of about 45 to 180 p.s.i.g. The air entrance and compressor sections may follow any one of several designs; and the compressor section may possess either an axial or a centrifugal compressor. If of the centrifugal type, the compressor may additionally possess either a single or a double entry. 7

From the compressor section, the compressed air flows into the combustion section where it is combined with a metered and atomized or prevaporized amount of fuel and its temperature increased by combustion of the fuel. It will be noted that the air flow in this section is such that only a relatively small amount of the air actually mixes with the fuel at the point of combustion. This portion of the air is generally referred to as the air supply. The weightratio of primary air to fuel is generally between 10 to 1 and 50 to 1.

Following the combustion of the fuel, the combustion products are almost immediately and intimately mixed with the remaining or secondary air. Thus, the combustion products are cooled from a combustion temperature of about 3500 to 4000 F. to an average temperature of about 1400 F. The latter temperature is dictated at the present time by the types of metals and metal alloys that are presently available for use within the turbine section of an engine.

The gaseous products of combustion and the excess air entering the turbine section from the combustion'section cause the turbine rotor or rotors to revolve and to drive the compressor in the compression section and also auxiliary equipment such as fuel pumps, lube oil pumps, generators, etc.

The gases leaving the turbine then flow into the tailpipe section from whence they vent to the atmosphere. The design of this section may vary considerably. For example, it may have either a single or double exit, and it may also be of the variable orifice or adjustable exhaust nozzle type. When the tailpipe is provided with an after burner, it is essential that a variable exhaust opening be provided to adjust for both normal and afterburning combustion conditions. The tailpipe section normally operates at temperatures of about 900 to 1400" F.

When after burner nozzles are provided in the tailpipe section, a diffuser is usually placed between the turbine section and the nozzles. This device serves to redistribute the gas flow in the tailpipe and to promote better combustion of the fuel issuing from the afterburner nozzles.

The combustion section may be any one of the conventional types as, for example, one that employs multiple combustion chambers (cans) or one that uses an annular combustion liner or chamber (a burner basket). In the first of these'types, the air flow is split upon leaving the compressor and equal portions sent to each can, where these portions are combusted with portions of the fuel. The combustion products are then recombined with secondary air and routed to the turbine section.

When the combustion section is of the burner basket type, the primary portion of the air is diverted from the main stream and directed toward the fuel injector within the basket where it burns with the fuel. The remaining or secondary air is then mixed with the products of combustion at a point prior to their entrance into the turbine section.

The turbine section of a jet engine may contain one or more turbine rotors and one or more stages. In addition, the turbine blades may be of the impulse and/ or reaction types and may or may not be shrouded. Associated with the turbine rotor blades are stator blades which direct the hot gases against the rotor blades.

One of the more serious problems associated with the use of boron containing fuels in turbo-jet aircraft engines results from the formation of boron oxide as a combustion product and its subsequent deposition on the surfaces of the combustion section, the turbine section, and the tailpipe section. For example, a pound of diborane produces 2.5 pounds of B 0 and a pound of pentaborane, 2.76 pounds. The glass-like boron oxide has an approximate melting point of 840 F. and has a high viscosity at turbine operating temperatures of about 1400 to 1600 F. Thus boron oxide deposits collect and flow along engine parts, including combustor walls and transition pieces, turbine rotor and stator blades, tailpipe walls, including afterburner parts, and variable-areav nozzles.

It has been proposed to minimize deposits of boron oxide in turbo-jet engines by avoiding the strong reverse flow that is usually designed into a high velocity combustor to provide flame piloting. The wide flammability limits .prevent impingement of boron oxide on the surfaces thereof. Various methods of providing the air film are available such as porous walls, louvers, step construction, etc. These proposals, however, have not been successful in alleviating the boron oxide deposition problem.

In accordance with the present invention, boron oxide deposits are minimized within the combustion, turbine and exhaust sections of an air-breathing jet-type aircraft engine operating on a boron containing fuel by introducing within the appropriate section a mixture of sodium and cadmium metals containing about 5 to 25 weight percent of sodium.

The metal admixture can be introduced within those sections where boron oxide is prone to deposit by any one or more of several known means including introduction within the air or combustion gas stream flow ing to the pertinent sections and introduction directly into the pertinent section. The form in which the metal admixture is injected, i.e. whether molten or as a finely divided solid, is not critical and the form most con- Patented Oct. 31, 1951 venient for introduction at a particular point is employed. For example, the metal admixture can be introduced within the combustion section by its incorporation into the secondary air stream from the compressor section in a.finely divided solid form or by filming the combustor walls with an air stream containing the finely divided solid. The metal admixture can be introduced within the turbine section, for example, by direct injec tion of molten metal through small holes in the stator blades or by transpiring the molten metal through a stator fabricated from a porous, sintered stainless steel. The metal admixture can be introduced within the exhaust section by direct injection into the gas stream flowing therein as a finely divided solid. Where an afterburner is employed, the metal admixture can be injected at or subsequent to the difiuser and also in the air stream supplied to the afterburner.

In order to test the efficiency of the metal admixture in minimizing the deposit of boric oxide on heated surfaces, a combustion and exhaust system simulating that of a jet-type engine was constructed and is shown in the attached drawing wherein FIGURE 1 is a side elevation and FIGURE 2 is an isometric of the combustion and exhaust system, FIGURE 3 is a section along line A-A of FIGURE 1, FIGURE 4 is a section along line BB of FIGURE 1, and FIGURE 5 is a section along line O-C of FIGURE 1.

In the drawing numeral 1 represents a section of four inch diameter stainless steel pipe about sixty-eight inches long and divided into four sections by flanges 2, 3, 4 and 5. Attached to pipe -1 by means of flange 6 was an eight inch diameter section of stainless steel pipe 7 about inches long defining the exhaust chamber.

The section of pipe 1 between flanges 2 and 3 had disposed therein an Inconel liner 8, defining the combustor, with an average diameter of 3.12 inches which tapered outwardly toward flange 3 one-eighth inch per foot and was two feet long. Liner 8 was perforated and contained eighty holes varying from three-sixteenths inch to three-eighths inch in diameter. Air entered the combustor through inlets 9 and 9a. The bulk of the air entered inlet 9 at 160 F. IP-4 fuel was injected into the combustor by means of inlet 10 and boron containing fuel was injected into the combustor by means of inlet 11. Inlet 11 was connected to a standard 80 hollow cone oil burner nozzle and inlets 9a and 10 were connected to a fuel-air nozzle which injected the mixture into the combustor by means of an annular opening around the oil burner nozzle. The fuel-air mixture was ignited by means of a high energy, high voltage, air-gap type spark plug 12.

The section of pipe 1 between flanges 3 and 4 was fourteen inches long. Two inches upstream from flange 4 was thermocouple bank 13 consisting of two thermocouples.

The section of pipe 1 between flanges 4 and 5 was ten inches long and defined the test Section which contained three Vycor-glass windows and a probe port located circumferentially around pipe 1 midway in the test section. Through the probe port was inserted deposition probe 114 which consisted of a section of onehalf inch stainless steel tubing with the base closed by welding and which contained three 0.016 inch diameter holes drilled along the leading edge. The leading edge had a projected deposition area of 1.5 square inches. Probe 14 was attached to pipe '1 by a stainless steel flare. Above probe 14 was positioned a reservoir 15 containing the molten metal admixture. Reservoir 15 was equipped with a wrap-around resistance heater and nitrogen pressure, usually 0.5 to 1.0 p.s.i., was ap lied to reservoir 15 by means of line 16. The flow rate of the molten metal was controlled by varying the nitrogen pressure applied to the reservoir;

-The section of pipe 1 between flanges 5 and 6 was ten inches long. Six and one-half inches downstream from flange 5 was thermocouple bank 17 consisting of eight thermocouples.

This combustion and exhaust system was employed in each of the following examples.

Example I Air flow through inlets 9 and 9a was established and IP-4 was injected through inlet 10 and burned in the combustor at the flow rate required to provide an exhaust gas temperature of 1500 F. measured by thermocouple bank 17. Operation with JP-4 alone was continued for a preheating period of five minutes. A mixture of trimethoxyboroxine and acetone (4 parts trimethoxyboroxine and 1 part acetone) was then injected into the combustor through inlet 11 at a flow rate of 0.0055 pound per second for intervals of three, four or five minutes, the JP-4 flow rate being adjusted to maintain a temperature of 1500 F. at thermocouple bank -17. At the end of the three, four and five minute intervals of trimethoxyboroxine flow, JP-4 combustion was continued for thirty seconds at which time the deposition probe 14 was removed and weighed. By this means it was determined that the boric oxide deposition rate on the probe was 0.06 pound per hour.

The probe was cleaned and replaced and the system again brought to preheat temperature with JP-4. A molten admixture containing cadmium and 10% sodium was charged to reservoir 15 and nitrogen pressure was applied until a transpirant flow of molten metal was established through the orifices in probe 14, as observed through the Vycor windows. The trimethoxyboroxine acetone how was then started, the JP-4 flow rate being adjusted to maintain a temperature of 1500 F. at thermocouple bank 17. The experiment was continued for about 8 minutes until all of the molten metal admixture had been expended. As observed through the Vycor glass windows, the molten metal admixture was effective in preventing boric oxide deposition on the probe. The flow of molten metal admixture was greater than the minimum requirement since it was observed that some of the molten metal did not flow over the probe because of a slight misalignment of the orifices. Excess pressure also caused some molten metal to leave the probe without flowing over the probe surface. The test conditions are summarized as follows:

Trimethoxyboroxine flow lb./sec 0.0055 Air flow lb./sec, 0.25 Stream temperature F 1500 Length of test minutes 8 Molten metal admixture lbs./hr 4 Example 11 Example 111 In a separate experiment carried out in a manner similar to that of Example I, molten cadmium metal was substituted for the sodium-cadmium admixture of Example I. The cadmium produced a cadmium oxide which mushroomed on the leading edge of the probe and in a short time was covered with boric oxide.

In two additional experiments carried out in a manner similar to that of Example I, mixtures of 10% sodium with 90% tin and 90% lead respectively were substituted for the sodium-cadmium admixture of Example I. Some activity was observed but neither admixture was as effective as the sodium-cadmium admixture of Example I.

The amount of metal admixture required to be introduced within the various sections of the engine in order to minimize the deposition of boric oxide varies with the about 5 to 25 weight percent of sodium in contact with surfaces upon which boron oxide would normally decharacteristics of the engine and the conditions under which it operates. For example, in ram-jets, where a lesser deposit problem exists than in turbo-jets, less metal posit in an amount suflicient to minimize boron oxide admixture is required. In general, however, the total deposition. 7

amount of metal admixture introduced within the various 5 2. The method of claim 1 wherein the mixture contains engine sectionstissufficient.totmaintainihetmposidcm 449% byuizeightzot WWW. i g g ifi desirably References Cited in the file of this patent 1. A method of minimizing boron oxide deposits UNITED STATES PATENTS within the combustion, turbine and exhaust section's efio 2,654,670 Davis, of a1. Oct. 6, 1953 an air-breathing jet-type aircraft engine operating on a fuel containing at least one material selected from the class consisting of boranes and organoboranes which con- OTHER REFERENCES sists of introducing within such a section a mixture consist- Pfoefl 6t The J 01111131 of p Y ing essentially of sodium and cadmium metals containing 15 January 1950, PP- 1 and PY In Sclefltlfic YJ 2,744,380 McMillan et a1 May 8, 1956 

1. A METHOD OF MINIMIZING BORON OXIDE DEPOSITS WITHIN THE COMBUSTION, TURBINE AND EXHAUST SECTIONS OF AN AIR-BREATHING JET-TYPE AIRCRAFT ENGINE OPERATING ON A FUEL CONTAINING AT LEAST ONE MATERIAL SELECTED FROM THE CLASS CONSISTING OF BORANES AND ORGANOBORANES WHICH CONSISTS OF INTRODUCING WITHIN SUCH A SECTION A MIXTURE CONSISTING ESSENTIALLY OF SODIUM AND CADMIUM METALS CONTAINING ABOUT 5 TO 25 WEIGHT PERCENT OF SODIUM IN CONTACT WITH SURFACES UPON WHICH BORON OXIDE WOULD NORMALLY DEPOSIT IN AN AMOUNT SUFFICIENT TO MINIMIZE BORON OXIDE DEPOSITION. 